In a wellbore, particularly in an oil or gas well, it is often critical to determine the depth at which the liquid gas interface is located so that the pressure at the formation face can be determined. While pressure or fluid depth can be measured with a wireline tool, this involves a great deal of surface equipment and is quite expensive. In addition, the wellbore can also expose the wireline tools to hostile CO.sub.2 and H.sub.2 S environments with bottom hole temperatures approaching 500.degree. F. The drawbacks involved with wireline testing have led to the development of acoustic ranging devices which generate a sonic impulse in the wellbore and then sense the acoustic reflections off various surfaces and objects, including the liquid gas interface.
Acoustic sensing devices have been developed which contain an explosive charge to generate the sonic event. An example of such a device is U.S. Pat. No. 3,915,256, issued on Oct. 28, 1975 to James N. McCoy. This device employs a common blank shotgun shell to generate the sonic event.
In a wellbore which has a substantial gas pressure, it is possible to use the gas pressure itself to create the sonic event. An example of a device of this type is U.S. Pat. No. 4,408,676, issued Oct. 11, 1983 to James N. McCoy. In this device, a valve is rapidly opened between a passage connected to the wellbore and a chamber at a lower pressure than the wellbore. This creates a sonic implosion which generates the sonic event.
While these devices and other devices in the industry have proved suitable in many applications, these devices are designed for use in wellbores having a pressure no higher than about 1500 psi. More recently, it has become desirable to sense the liquid gas interface in gas wells where the pressure may be of the order of about 15,000 psi. Also, in the past, the most successful test results have been made by sending a sonic impulse down the annulus between the casing and tubing therein. Therefore, reflections from internal tubing collars, especially close-makeup or flush couplings, could not be detected. A need exists for a device which is more sensitive than past instruments to detect flush connections and which is capable of operating at such elevated pressures while remaining economical and simple to use.